GH Actions: clean up

* Remove the now redundant Travis script.
* Remove the reference to the Travis script from `.gitattributes` and add the new `.github` folder.
* Update the "Build status" badge in the README to show the result of the GH Actions run instead of Travis.

Supersedes 54

Co-authored-by: Sam Reed <sam@reedyboy.net>

Author: jrfnl

.gitattributes

@@ -7,7 +7,7 @@
 #
 /.gitattributes export-ignore
 /.gitignore export-ignore
-/.travis.yml export-ignore
+/.github/ export-ignore
 /appveyor.yml export-ignore
 /box.json export-ignore
 /doc export-ignore

.travis.yml

@@ -1,7 +1,7 @@
 # PHP Parallel Lint
 
 [![Downloads this Month](https://img.shields.io/packagist/dm/php-parallel-lint/php-parallel-lint.svg)](https://packagist.org/packages/php-parallel-lint/php-parallel-lint)
-[![Build Status](https://travis-ci.org/php-parallel-lint/PHP-Parallel-Lint.svg?branch=master)](https://travis-ci.org/php-parallel-lint/PHP-Parallel-Lint)
+[![Build Status](https://github.com/php-parallel-lint/PHP-Parallel-Lint/actions/workflows/test.yml/badge.svg)](https://github.com/php-parallel-lint/PHP-Parallel-Lint/actions/workflows/test.yml)
 [![License](https://poser.pugx.org/php-parallel-lint/php-parallel-lint/license.svg)](https://packagist.org/packages/php-parallel-lint/php-parallel-lint)
 
 This application checks syntax of PHP files in parallel.